1. Field of the Invention
This invention relates to colored light emission in light emitting device structures for use in a variety of apparatuses.
2. Description of the Background Art
Organic light emitting devices (OLEDs) are an emerging technology that may soon replace liquid crystal displays (LCDs) in flat panel display applications due to their desirable characteristics including self-emissive high brightness, wide viewing angles, light-weight, and low power consumption. Recently, Sony previewed a prototype of an OLED-based display that is slightly thicker than a credit card and announced production to start in 2003. A display is made up of many tiny individual pixels (picture elements) where, an OLED represents one pixel. In a full-color display, each pixel contains one or all of the three color components, namely, red, green and blue (RGB).
An OLED consists of a transparent substrate, typically glass or plastic, coated with a transparent conducting material, such as Indium Tin oxide (ITO), one or more hole injecting and/or hole transporting layers (HTL), one or more electron transporting (ETL) and/or electron injecting layers and a cathode made up of low work function metals. The HTL or ETL may also have light emissive properties or a separate emitting layer may be sandwiched between the HTL and ETL.
Developing efficient and economical methods to manufacture RGB patterned pixels is one of the main issues concerning the realization of full-color flat panel displays. Several approaches have been developed to achieve full-color organic emissive displays. The first method consists of filtering white light with RGB band-pass filters. This technique results in a large reduction of the optical power from the white OLED. Thus the color-filtered OLEDs must be operated at high brightness/current density, which may accelerate degradation and shorten the lifetime of the device. Another method utilizes the conversion of blue light to green light and red light through a color converting layer comprising a fluorescent material and has been demonstrated with many variations (See U.S. Pat. Nos. 5,126,214; 5,294,870; 6,019,654; 6,023,371; 6,137,221; 6,249,372, all incorporated by reference herein). A major challenge of this method is the difficulty of finding a red fluorescent material with a high absorption coefficient in the blue wavelength region and having a high fluorescence in the red wavelength region. This method also results in reduced device efficiency during the color conversion process.
Yet another method used to achieve RGB emission is through the patterning of discrete RGB sub-pixels. This method has been demonstrated with the use of precise shadow masks (See U.S. Pat. No. 6,214,631, herein incorporated by reference). This patterning method has also been accomplished with a laser ablation technique (See U.S. Pat. No. 6,146,715, herein incorporated by reference) which is used to etch away undesired organic and electrode layers as a way to avoid using harsh photoresist chemicals to pattern discrete RGB pixels adjacent to each other on the same substrate. This approach is more advantageous than the others because the red, green, and blue OLEDs are individually optimized to achieve high device efficiencies at low power. Typically, three different OLED structures are used in order to optimize each color pixel, with a minimum of two different materials (host and dopant) for each of the primary colors. The use of several different types of material components during device fabrication may increase the risk for cross-contamination and would bring about a more complicated process for device fabrication.
Organic electroluminescent devices that include organic host materials and dopants are disclosed, for example, in the following patents and publications, which are all herein incorporated by reference: U.S. Pat. No. 3,172,862 to Gurnee et al; U.S. Pat. No. 3,173,050 to Gurnee; U.S. Pat. No. 3,710,167 to Dresner et al; U.S. Pat. No. 4,356,429 to Tang; U.S. Pat. No. 4,769,292 to Tang et al; U.S. Pat. No. 5,059,863; U.S. Pat. No. 5,126,214 to Tokailin et al; U.S. Pat. No. 5,382,477 to Saito et al; U.S. Pat. No. 5,409,783 to Tang et al; U.S. Pat. No. 5,554,450 to Shi et al; U.S. Pat. No. 5,635,307 to Takeuchi et al; U.S. Pat. No. 5,674,597 to Fujii et al; U.S. Pat. No. 5,709,959 to Adachi et al; U.S. Pat. No. 5,747,183 to Shi et al; U.S. Pat. No. 5,756,224 to Börner et al; U.S. Pat. No. 5,861,219 to Thompson et al; U.S. Pat. No. 5,908,581 to Chen et al; U.S. Pat. No. 5,932,363 to Hu et al; U.S. Pat. No. 5,935,720 to Chen et al; U.S. Pat. No. 5,935,721 to Shi et al; U.S. Pat. No. 5,948,941 to Tamano et al; U.S. Pat. No. 5,989,737 to Xie et al; International Publication No. WO 98/06242 (Forrest et al); C. W. Tang et al “Electroluminescence of Doped Organic Thin Films”, J. Appl. Phys. 65(9), May 1969, pp 3610–3616; C. W. Tang and S. A. VanSlyke, “Organic Electroluminescent Diodes”, Appl. Phys. Lett. 51(12), Sep. 21, 1987, pp. 913–915; C. W. Tang, “Organic Electroluminescent Materials and Devices” Information Display, October 1996, pp. 16–19; J. Shi and C. W. Tang, “Doped Organic Electroluminescent Devices with Improved Stability”, Appl. Phys. Lett 70(13) Mar. 31, 1997, pp. 1665–1667; Shoustikov et al, “Electroluminescence Color Tuning by Dye Doping in Organic Light-Emitting Diodes”, IEEE Journal of Selected Topics in Quantum Electronics, Vol. 4, No. 1 January/February 1998, pp 3–13; Baldo et al, “Highly Efficient Phosphorescent Emission from Organic Electroluminescent Devices”, Nature, Vol. 395, Sep. 10, 1998, pp 151–153; O'Brien et al “Improved Energy Transfer in Electrophosphorescent Devices”, Applied Physics Letters, Vol. 74, No. 3, Jan. 18, 1999, pp. 442–444.